LV2 plugin support

Thanks Aaron -- we actually already have a proof of concept for LV2 support :). I could have sworn we already had a bug report open but it seems we don't. Regardless, this is indeed on our radar!

I believe the latest code for LV2 support is here:
https://github.com/daschuer/mixxx/tree/lv2_support2

This is in progress but stalled: https://github.com/mixxxdj/mixxx/pull/1240

There is quite a bit of work that needs to be done on the effects system before LV2 effects are practically usable. See also:
https://bugs.launchpad.net/mixxx/+bug/1653325
https://bugs.launchpad.net/mixxx/+bug/1653140

My plan was to have it in 2.2.
It is already usable as it is. If we do not merge this soon, it will start to rott even more, which is unfair to the original developer who has build a working solution.

I already have enough (and maybe more than enough) plans for myself for 2.2. Are you planning to do it for 2.2? If not, then let's not keep it on the 2.2 milestone. Even still, I think doing this for 2.2 would likely be rushing it. I suspect there will be complicated issues that arise when interfacing with external software plugins that we have not thought about yet. I'd like to avoid burning out our small team of volunteer developers, so I think it would be wise to lay the prerequisite groundwork for usable implementations of external plugin support in 2.2 then deal with the details of implementing the external plugin support in 2.3. Perhaps we can revisit this discussion half way through the new feature development time for 2.2 when 2.1 final is released.

Since it is almost ready, it should be doable for 2.1.

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/8402